A. Social Environment Effects on Play. Data analysis was completed for this study which investigates the immediate effects of manipulating the play behavior of young male squirrel monkeys by restricting the pool of available play companions. The 12 subject males maintained high levels of play activity with four different classes of play partners (including classes normally avoided by them) by adapting their play to each class of partner. This indicates a high degree of social awareness, as well as a strong motivation to play. Our previous studies suggested a major role for normal play interactions in the development of flexibility and resilience in social skills. The present findings suggest that maladaptive personality types like "bullies" and "sissies" may result when play is restricted to inappropriate partners for extended periods. We have concluded that play is of such value to developing monkeys that it will persist even under suboptimal conditions. These studies (which manipulate play without eliminating social contact) have reached a point where definitive studies on the function of play are possible. B. Vocalizations Used in Play. Our previous study found that the abundant vocalizing of squirrel monkeys during play had very limited communicative significance for the playing animals themselves. We have recently begun to test an alternative function--that such calls signal nearby adults to the presence of play activity. Benefits of such a message include reassurance that the young are engaged in a harmless activity and alerting adults to increase their vigilance for predators while the young are preoccupied in play. Noisy play, which is unusual in animals, may in fact attract the attention of predators and thereby increase the risk involved in play. We are now measuring changes in adult vigilance behavior as a function of play vocal activity.